Horseshoes, in general, have not changed much in the last 1,000 years. Horseshoes are usually used to protect a horse's hoof when the horse is put to productive or useful work, or engaged in competitive events and hard training. There are many types of horseshoes with most being made of metal—primarily steel or aluminum—and fastened directly to the hoof with nails.
For instance, most farriers will use six to eight nails per shoe, but will not insert a nail beyond the midline thereby allowing the heels of the hoof to move laterally across the horseshoe.
Although the vast majority of horseshoes are still applied with nails, more recently, in the last 30 years, progress has been made in successfully fastening metal horseshoes—generally aluminum—to the hoof using glue. Directly gluing horseshoes to a horse's hoof may become necessary for a horse, which has a damaged hoof—broken, cracked, thin, brittle or shelly.
However, a potential drawback with directly gluing a horseshoe to horse's hoof is that the horseshoe is glued to the entire hoof perimeter beyond the midline to generate an adequate bond strength. Keep in mind, that a horse typically weighs over a 1,000 lbs. and can run over 30 mph, cut at 45°, and jump over a five-foot object. This is why the horseshoe is usually glued to the entire perimeter of the hoof. Otherwise, if the horseshoe is not adequately bonded to the hoof, it will either fall off the hoof, become dislodged or loose, and may injure the horse and its rider.
But gluing the entire hoof perimeter past the midline to the horseshoe has drawbacks. For instance, if the heels of the horse's hoof are glued to the shoe, the heels cannot laterally expand and contract as it loads and unloads, and will not naturally bio-mimic the action of an unconstrainted hoof. Thus, gluing the heels of a horse's hoof to the horseshoe is often counterproductive, because the heel cannot move naturally on the heel of the horseshoe, often resulting in lameness, pain and soreness to an already sore or damaged hoof.
Another drawback with aluminum horseshoes, is that they don't offer shock attenuation. This can result in aggravating soreness in the horse's hoof. And although aluminum shoes are generally lighter than steel horseshoes, aluminum shoes are about 40% heavier than plastic horseshoes.
That is why many farriers select plastic horseshoes (i.e., urethane or polyurethane) over aluminum or steel. Put differently, a well-designed plastic horseshoe offers substantial shock attenuation over steel or aluminum horseshoes. In addition, plastic horseshoes are about 70% lighter than steel shoes, and are almost half as light as aluminum. Many plastic shoes also offer adequate support and grip to the horse, when it performs work.
But plastic horseshoes suffer from many of the same problems as aluminum shoes when it comes to attaching the horseshoe to directly the horse's hoof using adhesive bonding. That is, like aluminum shoes, plastic horseshoes are often applied to horse's hoof in a non-traditional manner by “glueing” the shoe directly to the bottom of the horse's hoof without any nails. Farriers may trim the hoof, shape the shoe, and then apply to the hoof wall with an adhesive. Two potential common adhesives may be used to attach the horseshoe to the hoof: (1) urethane adhesive, or (2) an acrylic adhesive.
While urethane adhesives grab a urethane-based horseshoe quickly and reliably, farriers generally prefer to use acrylic adhesives—such as a methyl-methacrylate—because it tends to generate a more reliable bond between a horse's hoof and the horseshoe than a urethane adhesive.
That is, methacrylate adhesives are more robust than urethane adhesives. For instance, methacrylate adhesives tolerate more surface contamination and a bit more moisture in the hoof than urethane adhesive do. This is why farriers and equine vets—who work in less than clean environments when shoeing, and can't easily control the bond-surface condition—prefer methacrylate adhesives over urethanes.
Although methacrylate adhesives will bond well to a hoof (an organic material) it generates varying bonding strengths depending on the chemistry of the plastics the methacrylate is attempting to “grab” (i.e., bond to). This may result in plastic horseshoes that are not effectively bonded to the horse's hoof, and may fall off, or become partially dislodged, which can injure a horse or its rider.
To avoid this problem, most farriers glue the heels of the hoof to the plastic horseshoe to achieve a total overall higher bonding strength, because more surface area generally equals higher strength with adhesives. Again, this negatively constrains the natural movement of a horse's hoof, and may result in heel bruises as a result.
There are other problems with plastic horseshoes. For instance, many new plastic horseshoes, which are injection molded are only available in a limited number of shapes and sizes; most likely due to the high cost of producing an injection die for each size and each shape of horseshoe. Thus, plastic horseshoes that are injection molded are usually expensive, and not readily available in different shapes and sizes, due to expense.
Further, because injection molding produces a part that is thermos-plastic, the horseshoe is not highly resistant to abrasion. This may result in a horseshoe that wears prematurely or unevenly.